Building the cerebral cortex requires the spatial and temporal orchestration of molecular signals across neural progenitor cells. Each of these cells is bathed in cerebrospinal fluid (CSF), a medium rich in health- and growth- promoting factors whose composition changes profoundly across days during brain development. The choroid plexus (ChP) is likely to strongly contribute to this dynamic developmental regulation of CSF composition, but the underlying mechanisms are not understood. Here, we will use a suite of modern tools to test the hypothesis that elevations in calcium concentration in embryonic ChP epithelial cells in response to extrinsic cues is critical for regulating synthesis and secretion of signaling factors into the CSF. Calcium activity regulates transcription and secretion in diverse cells ranging from neurons to salivary gland epithelia. We identified receptors expressed in ChP epithelial cells that mediate calcium entry into the cell, such as the TRPM3 receptor, or that drive calcium release from internal stores, such as the 5HT2C receptor (Htr2c) for serotonin (5- HT, 5-hydroxytryptamine). Our preliminary data suggest that 5-HT regulates embryonic ChP calcium activity, transcription, and vesicle release, and that intact signaling via the 5HT2C receptor in ChP is essential for normal cerebral cortical development. Further, maternal delivery of a selective agonist for the 5HT2C receptor triggers rapid, robust immediate early gene expression in embryonic ChP. Thus, we propose that ChP calcium activity triggered by signals that activate the 5HT2C receptor (and other receptors such as TRPM3) regulates ChP transcription and secretion of signaling factors into the CSF and controls body-brain and maternal-fetal interactions, thereby tightly orchestrating cerebral cortical development. To test this, we will first investigate the effects of 5HT2C and TRPM3 receptor activation by direct application of ligand to the apical surface of epithelial cells during calcium imaging in ChP explants (Aim 1a) and by stimulation of the basal surface following intraperitoneal (IP) delivery of ligands to the maternal dam during in vivo two- photon calcium imaging of attached embryos (Aim 1b). We then assess the transcriptomic impact of such stimulation (Aim 1c), and test whether embryonic ChP calcium activity via activation of 5HT2C receptors and other receptors triggers vesicle release and protein secretion in vitro (Aim 2a) and in vivo (Aim 2b). We will also analyze the effects of disruption of ChP receptor signaling on cortical development (Aim 3a). Disrupted CSF volume, composition, and ventricle formation are common to many neurological conditions. Perturbations in maternal-fetal health can disrupt brain development, in part via dysregulation of serotonergic and other signaling pathways. Our findings provide a novel platform for elucidating the mechanisms underlying dysregulation of brain development by environmental perturbations, thereby providing a conceptual and technical roadmap for future studies on how activity in ChP epithelial cells influences gene expression and protein secretion, with consequences for many neurodevelopmental diseases.